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BQ76940: Most upper Balancing Resistor burns randomly through with external PMOS and drains 1 cell

Steffen Dase
Prodigy 155 points
Part Number: BQ76940
Other Parts Discussed in Thread: BQ78350

Hello,

we developed a 13s BMS with the bq76940 and the companion IC bq78350.

The companion IC is managing the balancing.

In an big field test we have the problem, that on nearly 1/3 of 100 BMS the most upper balancing resistor is defective (burned away from overstress). This was discovered, when deeply discharged batteries (2,7V-3V) stand for some days and couldnt be charged anymore. The check of the cellvoltages showed, that one of 13 cells was deeply discharged (<1V), the rest had 2,7V-3V. Most of the time was the deep discharged cell the most upper cell, where the balancing resistor is defective. The balancing Mosfet is ok. The resistor is a thick film resistor with 0,5W an 47R.

Do you have any idea why the cells have been discharged and the resistor gets broken?

  • 0
    TI__Guru* 80843 points
    Hi Steffen,

    This is a strange issue. The schematic looks okay to me except the filter capacitors on the cells are very small and are not within the range of the datasheet. The small value may allow larger voltage transients to pass through which may be causing damage.

    Also, this is not related to the issue you are seeing, but the recommendation for the VC1 Cc cap (C29) is for it to connect between VC1 and GND, not VC1 to VC0. This could slight the affect the voltage readings on the lowest cells.

    Best regards,
    Matt